1. Field of the Invention
This invention is related to semiconductor light-emitting diodes (LEDs) and in particular presents optimized optical designs aimed at improving the luminous efficacy of white-light emitting diodes (WLEDs) used for lighting applications.
2. Description of the Related Art
Currently, there exists a multitude of WLED packaging configurations in the state of the art. The term packaging encompasses a broad technical scope. With respect to LEDs it refers to all the fabrication steps which follow wafer processing: dicing of LED chips, transfer of these onto headers or supports to provide electrical injection and heat-sinking, integration of secondary light-emitting species, and encapsulation with transparent materials to enhance light extraction and allow device protection and passivation (this sequence of steps could be performed in different order).
In the following, the term LED die is used to refer to a semiconductor chip, which includes electroluminescent primary light-emitting species (such as quantum wells or any other type of semiconductor heterostructures). The term phosphors refers to the optically pumped secondary light-emitting species, without loss of generality.
Out of the large number of ways of packaging WLEDs, there are mostly two main configurations for phosphors integration: phosphors-on-chip and remote-phosphors configurations.
As FIG. 1 shows, in the phosphors-on-chip configuration 100, the phosphors 102 are placed in the direct vicinity of the semiconductor LED die 104, either as powders coating the chip 104 or as mixtures with resins surrounding the chip 104, with different concentrations and geometrical dimensions. The LED die 104 is usually fixed on a reflecting header or LED cup 106, providing electrical injection and heat-sinking, and embedded in a transparent epoxy 108 (resin, silicone, etc.).
The use of a transparent epoxy 108 allows an increase in the light extraction efficiency of the device 104 because transparent epoxies have a higher index of refraction (n) than air in the near-ultraviolet, visible, and infrared wavelength ranges. Light which is emitted from within the high-index semiconductor chip 104 (usually n>2 for most semiconductors) can escape outside only if the light's angle of incidence inside the chip 104 is within the light escape cone, that is, below the critical angle for total internal reflection (TIR) θc. θc depends on the index of refraction of the medium which surrounds the LED die 104, nout, and the refractive index of the LED die 104, nin: θc=arc sin(nout/nin). The value of θc increases from 24° to 35° as the medium exterior to a GaN LED die 104 (nin=2.5) is changed from air (nout=1) to a common transparent epoxy 108 (nout=1.45).
For this increase in light extraction to occur, it is necessary that (1) the LED chip 104 and the epoxy 108 be in close contact (if even a thin layer of air or vacuum separates the LED chip 104 from the epoxy 108, the potential increase in light extraction is cancelled), and (2) the interface 110 between epoxy 108 and external medium (usually air) be curved or shaped, such that the greatest portion of the light rays extracted from the LED die 104 and phosphors 102 impinge onto this interface 110 at incidence angles much smaller than the critical angle for total internal reflection: θc˜43° for epoxies 108 with an index of refraction ˜1.45.
The whole LED die+cup+resin+phosphors assembly can therefore be placed in a hemispherical or dome-shaped material 112 (an optic), which can be made out of a transparent epoxy or optical glass. With this shape, most light rays are incident at nearly 0° (θc˜43°), the angle at which reflectance is minimized.
However, in the phosphors-on-chip configuration 100, light rays emitted from the phosphors 102 downwards (that is, emitted towards the LED die 104 and header or LED cup 106) are partly absorbed by the LED metal contacts and the doped semiconductor layers necessary to electrically inject the primary emitting species included in the LED chip 104. Indeed, in order for these light rays to escape, they must propagate downwards across the LED die 104, be reflected upwards by a sufficiently reflective layer (either included on the bottom surface of the LED die 104 or on the LED cup 106), propagate across the LED die 104 once again, and then through the phosphors layer 102, without being absorbed. In addition, light rays emitted by the phosphors 102 upwards, as well as the primary light extracted from the LED die 104 used to optically pump the phosphors 102, undergo multiple reflection, refraction, and scattering events, due to the presence of large phosphor particles (usually larger than 5 μm in diameter), wherein the phosphor particles have indices of refraction (n˜1.75 for rare-earths-doped YAG phosphors) which are usually different from the refractive index of the matrix in which the phosphor particles are embedded (n˜1.45). These scattering events increase the probability of absorption of light. Finally, in this configuration 100, phosphors 102 are in direct contact with the high temperature of the LED junction 114 under operation, which can reach temperatures larger than 150° C. At such elevated temperatures, the degradation rate of phosphors 102 is increased and their internal quantum efficiency is usually reduced.
These negative effects are partly eliminated by using the remote-phosphors configuration 200. In this configuration 200, the phosphors 202 are separated from the LED die 204, that is, they are placed at least 200 μm away from the upper surface 204s of the LED die 204, as depicted on FIG. 2. This configuration 200 allows an increase in the overall luminous efficacy of WLEDs, by reducing the probability of light absorption caused by scattering and absorption by metals and doped semiconductor layers. In addition, this configuration 200 places the phosphors 202 away from the region of elevated temperature, which is in the vicinity of the LED die 204 under operation. The resin 206, LED cup 208, and optic 210 are also shown in FIG. 2.
There are different possibilities for phosphors application in a remote phosphors configuration. FIG. 3 shows an example of a remote phosphors configuration 300, wherein the phosphors 302 are coating the dome-shaped light-extracting optic 304. The LED die 306, resin 308, and LED cup 310 are also shown in FIG. 3.
The previous configurations, and all of their possible geometries, are not restricted to be used in combination with the dome-shaped light-extracting optic 112, 210, or 304. Inverted-truncated cones can also be used instead to obtain similar light-extraction performance. In FIG. 4, the phosphors 400 are placed on top of an inverted-truncated cone-shaped optic 402, which could be formed from a resin or optical glass, while in FIG. 5, the whole assembly 404 of FIG. 4 (comprising the phosphors 400, optic 402, LED die 406 and LED cup 408) is capped by another dome-shaped optic 410. The use of such truncated cone 402 is a possible alternative to the use of hemispherical-shaped optics, because light rays extracted from the LED die 406 impinge on the sidewalls 412a, 412b of the cone 402 at angles larger than θc, and hence are totally internally reflected and can escape upwards through the upper surface 414 of the optic 402.
Although the previous geometries of the remote-phosphors configuration can help to reduce light absorption, there is still a need in the art for improving light extraction efficiency by making use of alternative packaging geometries.
Indeed, when phosphors 202, 400 are placed inside an LED cup 208, or adjacent an inverted cone 402 made of resin, as shown in FIG. 2 and FIG. 5 respectively, the phosphors 202, 400 are embedded in and/or surrounded by a material 206, 402, 210, and 410 with index of refraction of about 1.45. The symmetry of this configuration implies that light emitted from inside the phosphors layer 202, 400 propagates in nearly equal amounts upwards and downwards. Therefore, about 50% of the light emitted by the phosphors 202, 400 must undergo reflection at the bottom 208b of the LED cup 208 before having the possibility to escape outside. Part of this light is therefore absorbed in the process. Actually, the amount of light emitted by the phosphors 202, 400 downwards is slightly larger than that emitted upwards; indeed, the primary light rays (i.e. rays emitted by the LED die 406, 204) are incident from the bottom 416 of the phosphor layer 202, 400 (i.e. the part of the phosphor nearest the LED die 204, 406), and therefore more light is emitted by the phosphors located near the bottom 416 of the phosphor layer 202, 400 than those phosphors located near the top 418 of the phosphor layer 202, 400. As a consequence, the scattering of secondary light (i.e. light emitted by the phosphor 202, 400) propagating upwards is larger than that of secondary light propagating downwards.
With the other remote-phosphors geometry (as shown in FIG. 3 and FIG. 4), the situation is nearly similar, and in addition another complication stems from the fact that a smooth upper interface separating the phosphors layer and air is present near the phosphors. As FIG. 6 shows, the smooth interface 600 only allows the light rays 602 (emitted by a light source 604 such as a phosphor particle) incident at angles smaller than θc 606 inside the escape cone 608 to be extracted in the external medium 612 (usually air). The rest of the light rays 614, 616 are either totally internally reflected (totally internally reflected secondary light ray 614), or propagate downwards (transmitted secondary light ray 616) through transparent interfaces, such as 618, back towards the internal medium/optic 620 and the LED die 622, in useless directions, and towards regions where the probability for light absorption is not negligible. FIG. 6 also shows a possible trajectory for a transmitted primary light ray 624 emitted by the LED die 622, and transmitted into the phosphor layer 626 where the ray 624 interacts with a light source 604 such as a phosphor particle inside the phosphor layer 626. Thus, there is a need in the art for improved packaging configurations to enhance the light extraction from phosphor layers, for example. The present invention satisfies this need.